US Congress has formally validated Joe Biden's presidential election victory on a day that saw a time-honoured ceremony become a nightmare of unprecedented political terror.
The House and Senate certified the Democrat's electoral college win early on Thursday local time, after a violent throng of pro-Trump rioters spent hours running rampant through the Capitol.
Vice-President Mike Pence, in declaring the final vote totals behind Mr Biden's victory, said this "shall be deemed a sufficient declaration of the persons elected president and vice-president of the United States".
After claiming he would "never concede" during a rally hours earlier, President Donald Trump admitted defeat in the November 3 election for the first time, following the vote count.
The outcome had never been in doubt, but had been interrupted by rioters who forced their way past metal security barricades, broke windows and scaled walls to fight their way into the Capitol building.
A woman was fatally shot, windows were bashed and the mob forced shaken members of Congress and aides to flee the building, shielded by Capitol Police.
The rampage began shortly after Mr Trump repeated his unfounded claims of election fraud to thousands of rallying demonstrators he'd invited to Washington.
Thirteen Republican senators and dozens of GOP representatives had planned to force debate and votes on perhaps six different states' votes.
The assault on the Capitol made some Republicans squeamish about trying to overturn Mr Biden's win, and challenges were lodged only against Arizona and Pennsylvania. Both efforts lost overwhelmingly.
Mr Biden defeated Mr Trump by 306-232 electoral votes and will be inaugurated January 20. (ABC)